Question

(a) Draw the ray diagram showing the formation of image by a compound microscope.
(b) Show that in order to achieve large magnification in a compound microscope the magnitude of focal length of objective and eye piece should be small.

Answer 1

(A) : Compound microscope is consist of two convex lenses, one object of very small focal length with short aperture and one eyepiece, $E$ of moderate focal length and large aperture. The ray diagram showing the formation of image by a compound microscope is shown in the diagram.

(B) : The Magnifying power, $M=m_e\times m_o$ where $m_e$ and $m_o$ are the individual magnifying powers o objective and eye lens.

$M=\frac{v_0}{u_0}(1+\frac{D}{f_e})$--------- (1) when the final image is at near point.

$M=\frac{v_0}{u_0}\times \frac{D}{f_e}$-----------(2) when the final image is at the infinity.

From the equations (1) and (2) it is clear that for achieving a large magnification the focal length $f_e$ must be small. Hence the answer.